Why Heatwaves Can Kill — and How to Protect Yourself

An estimated 2,700 people died in England and Wales during the record-breaking heatwaves of May and June 2026 — and roughly 42% of those deaths, about 1,140 people, would not have happened without human-caused climate change. The most useful thing to understand about that number is what it isn't: it isn't a tally of people who collapsed with heatstroke.

TL;DR

• An estimated 2,700 excess deaths occurred in England and Wales across the May (~550) and June (~2,200) heatwaves — and the UK's hottest months hadn't yet arrived. The previous worst full summer on record was around 2,800–2,985 deaths.
• 42% of those deaths are attributable to human-caused climate change — deaths that would not have occurred in a pre-industrial climate.
• Most heatwave deaths are cardiovascular, not heatstroke. In one analysis of two heatwaves, cardiovascular disease was the primary cause in 51% and 64% of deaths. Heat-related illness itself contributes comparatively little.
• Heat is a cardiac stress test. To cool you, your heart pumps more blood to the skin while sweating drains fluid volume — a combination that strains a heart already under pressure.
• For every 1°C rise in temperature, cardiovascular deaths rise about 4% in the over-65s. Across Europe's June heatwave, over 9,000 of the 10,000+ excess deaths were in the 65+ age group.
• Most deaths don't happen in dramatic red-alert conditions. UKHSA data shows substantial mortality even during yellow alerts — the ordinary hot spells nobody worries about.
• The protective actions are unglamorous and effective: keep the home cool, hydrate before you're thirsty, avoid exertion in peak heat, know your medication risks, and check on people who live alone.

The Deaths You Don't See

If 2,700 people had died in a flood, it would dominate the news for a fortnight. Because they died in a heatwave, most people didn't notice — and the reason for that is worth understanding, because it explains almost everything about why heat is so dangerous.

Heat kills invisibly. There is rarely a body on a beach or a dramatic collapse in the street. Instead, an elderly woman with existing heart disease has a heart attack on the third night of a hot spell. A man with chronic kidney problems is admitted with acute kidney injury. Someone with COPD struggles to breathe in the hot, polluted air and doesn't recover. On the death certificates, these are cardiac arrests, strokes, renal failures, respiratory failures. Heat appears nowhere.

This is why heat-related deaths are calculated as excess deaths — the number of extra deaths occurring on hot days compared with normal summer days. It is a statistical fingerprint rather than a body count, and it is the only honest way to capture what heat actually does. The 2,700 figure comes from exactly that method, applied by researchers at the London School of Hygiene & Tropical Medicine, Imperial College London and the Met Office.

The invisibility is not a technicality. It's the reason heat is consistently under-rated as a health threat, and the reason so few people take a "yellow" heat alert seriously.

What Heat Actually Does to the Body

To understand why cardiovascular deaths dominate, you need to understand what your body does when it gets hot — because the cooling system itself is what causes the strain.

Your core temperature must stay around 37°C. When the environment heats up, the body has two main tools to shed heat, and both put load on the cardiovascular system.

First, it redirects blood to the skin. Vessels near the surface dilate so that warm blood can flow close to the air and dump heat. But that blood has to be moved, which means the heart must pump harder and faster to maintain circulation to everything else while also servicing this new demand at the surface. Cardiac output rises. The heart is, in effect, put on a treadmill it cannot step off.

Second, it sweats. Evaporating sweat is enormously effective — it can increase heat loss from the body by as much as ten times, which is what takes the pressure off the heart. But sweating drains fluid and salt. As you lose fluid, blood volume falls, and the heart is now trying to circulate a smaller volume through a wider network of dilated vessels. Blood pressure can drop, causing dizziness and fainting. And the blood itself changes: fluid loss raises plasma viscosity, increases red blood cell and neutrophil counts, and triggers the release of extra platelets into circulation. In plain terms, the blood thickens and becomes more prone to clotting — which is precisely the mechanism behind heart attacks and strokes.

Put those together and you have a body under sustained cardiovascular stress, with thickened blood, for days on end. For a healthy 30-year-old, this is uncomfortable. For an 80-year-old with existing coronary artery disease, it can be the thing that tips a manageable condition into a fatal event.

Why It's the Heart, Not Heatstroke

The evidence on this is clear and consistently under-reported. During a heatwave, most of the excess deaths are not directly heat-related illnesses — they are cardiovascular in origin.

An analysis of deaths across two heatwaves in Milwaukee found cardiovascular disease was the primary cause in 51% and 64% of deaths respectively. Heatstroke, heat exhaustion and the other classic heat illnesses contributed only moderately to the overall toll. Respiratory and cerebrovascular deaths made up much of the rest.

The pattern holds across the research. Heat exacerbates the conditions people already have. Someone with heart disease finds it harder to maintain adequate blood flow, leading to chest pain or heart failure. Someone with COPD or asthma struggles as hot air, air pollution and dehydration combine to inflame the airways. Someone with kidney disease or diabetes tips into acute kidney injury as dehydration overwhelms kidneys already working at reduced capacity.

The numbers are stark: for every 1°C rise in temperature, cardiovascular deaths increase by about 4% in people aged 65 and over. And heat doesn't act alone — under high air-pollution conditions, the cardiovascular mortality risk from heat is nearly double what it is under low pollution. Heatwaves and poor air quality tend to arrive together.

This reframing matters practically. If you think of heat as something that gives you sunstroke, you'll protect yourself by staying in the shade at noon. If you understand that heat is a multi-day cardiac stress test, you'll think differently about the 78-year-old with a heart condition living alone in a top-floor flat.

Who Actually Dies

The mortality is overwhelmingly concentrated, and knowing where means knowing who to protect.

Age dominates. In Europe's June 2026 heatwave, over 9,000 of the 10,000+ excess deaths were among people aged 65 and over. Older bodies are worse at this for concrete physiological reasons: the sweating response declines with age, both in rate and in output per gland, so evaporative cooling is less effective and more heat is stored. The ageing heart also struggles to increase stroke volume under heat stress the way a younger heart does, meaning older people rely on a greater proportion of their heart-rate reserve just to keep up. Less cooling, more cardiac strain — the worst possible combination.

Existing conditions multiply the risk. Cardiovascular disease, respiratory conditions, kidney disease, diabetes, Parkinson's, dementia, obesity, and severe mental illness all raise vulnerability.

Medications matter more than people realise. Drugs that affect heart or kidney function, or that impair the ability to sweat, or that affect cognition and behaviour, all increase risk. This includes some very commonly prescribed medications — diuretics, some blood pressure drugs, some psychiatric medications. This is not a reason to stop taking them; it is a reason to be more careful in the heat and to ask your GP or pharmacist about your specific prescriptions.

Isolation is a killer. Living alone, being socially isolated, or depending on others for daily activities are all significant risk factors — not for physiological reasons, but because there is nobody to notice things going wrong.

Environment matters. Top-floor flats, poor-quality housing without ventilation, homelessness, and outdoor occupations all mean more exposure and less escape.

The Alert Nobody Takes Seriously

Here is a finding that should change how you think about hot weather.

UKHSA data shows that a substantial number of heat-associated deaths occur during yellow Heat-Health Alerts — not just the dramatic amber and red ones. In the summer of 2023, the highest average number of deaths per day during any heat episode happened during a yellow alert in early August.

This makes sense once you understand the mechanism. It doesn't take 40°C to put sustained strain on a compromised heart. A few consecutive days in the low thirties, with warm nights that give no recovery, is entirely sufficient. And because a yellow alert doesn't feel like an emergency, nobody changes their behaviour, nobody checks on the neighbour, and the deaths accumulate quietly.

The lesson is that the danger isn't only in the record-breaking extremes. It's in the ordinary hot spell that everyone enjoys.

How to Protect Yourself

The genuinely encouraging part: most heat harm is preventable, and the actions required are simple and unglamorous. They just have to be taken before things go wrong, not after.

Keep the indoor space cool. This matters more than anything you do outdoors, because that's where vulnerable people spend the heat. Close curtains and blinds on sun-facing windows during the day. Open windows at night when the outside air is cooler than inside — but keep them shut during the day if it's hotter outside. Identify the coolest room in the home and use it, particularly for sleeping. If you have air conditioning or a fan, use it — though note that a fan blowing air hotter than body temperature can make things worse rather than better, so pair fans with an actual cooling method. Our guide to how to stay cool in a heatwave covers the practical steps in more depth.

Hydrate before you're thirsty. Thirst is a lagging indicator, and it becomes less reliable with age. Drink regularly through the day rather than waiting for the signal. Water is fine; if you're sweating heavily for long periods, replacing salts matters too. Go easy on alcohol, which worsens dehydration. If your doctor has restricted your fluid intake for a medical reason, follow their advice rather than this.

Avoid exertion in peak heat. The hottest part of the day is roughly 11am to 3pm. If you exercise, walk the dog, garden, or work outdoors, shift it to early morning or evening. Exertion adds internally-generated heat on top of the environmental load, and it's the fastest route from "hot" to "in trouble."

Cool the body directly. Cool showers, damp cloths on the neck and wrists, and wetting the skin all work — remember, it's evaporation that does the cooling, so wet skin is doing the same job sweat does, without the fluid loss.

Know your medications. If you take drugs for blood pressure, heart conditions, kidney problems, diabetes, or mental health, ask your GP or pharmacist whether they raise your heat risk and whether anything should change during a hot spell. Never stop a prescribed medication on your own. If you have hypertension, our piece on managing high blood pressure in the heat walks through the specific considerations.

Check on people. This is the single highest-value thing most readers can do, because you are probably not the person most at risk — but you likely know someone who is. An elderly relative or neighbour, particularly one living alone, in a top-floor flat, or with a heart or lung condition. A phone call, a knock on the door, a quick look at whether their home is stifling. The isolation risk factor exists precisely because nobody checks.

Take yellow alerts seriously. Sign up for Heat-Health Alerts if you or someone you care for is vulnerable. Act on them, even the mild ones — the data says that's where a lot of the deaths are.

Know the red flags. Heat exhaustion — heavy sweating, pale clammy skin, weakness, dizziness, nausea, headache — needs cooling and fluids, and should improve within 30 minutes. If it doesn't, or if someone has hot dry skin, confusion, a very high temperature, or loses consciousness, that is heatstroke: a medical emergency. Call 999.

Frequently Asked Questions

How many people died in the UK heatwaves of 2026?

Research published in July 2026 by LSHTM, Imperial College London and the Met Office estimates around 2,700 heat-related deaths in England and Wales across the May and June heatwaves — roughly 550 in May and 2,200 in June. Approximately 42% of these (about 1,140 deaths) are attributed to human-caused climate change. Notably, these occurred before the UK's traditionally hottest months.

How do heatwaves actually kill people?

Mostly through cardiovascular strain, not heatstroke. To cool itself, the body diverts blood to the skin (making the heart work harder) and sweats (reducing blood volume and thickening the blood). This combination stresses the heart and increases clotting risk. Most excess deaths in heatwaves are heart attacks, strokes, and worsening of existing heart, lung and kidney conditions — one analysis found cardiovascular disease was the primary cause in 51–64% of heatwave deaths.

Who is most at risk in a heatwave?

People aged 65 and over — who accounted for over 9,000 of the 10,000+ excess deaths in Europe's June 2026 heatwave — along with those with heart, lung, kidney conditions, diabetes, dementia, or Parkinson's; people on certain medications; those living alone or socially isolated; young children; outdoor workers; and people in poor-quality or top-floor housing without ventilation.

Why do older people cope worse with heat?

Two physiological reasons. Sweating declines with age — both the rate and the output per gland — so evaporative cooling is less effective and more heat is stored in the body. And the ageing heart is less able to increase stroke volume under heat stress, meaning older people rely on more of their heart-rate reserve just to maintain circulation. Less cooling plus more cardiac strain is a dangerous combination.

Do I need to worry about a yellow heat-health alert?

Yes. UKHSA data shows substantial numbers of heat-associated deaths occur during yellow alerts, not just amber and red ones — in summer 2023, the highest daily death rate of any heat episode occurred during a yellow alert. It doesn't take extreme temperatures to strain a compromised heart; a few consecutive warm days with warm nights is enough.

What's the single most useful thing I can do?

Check on someone vulnerable. Social isolation is a major risk factor precisely because nobody notices when things go wrong. If you know an elderly relative or neighbour, particularly one living alone or with a heart or lung condition, a phone call or a visit during a hot spell — checking their home isn't stifling and they're drinking enough — is genuinely one of the highest-value protective actions available. And whether you're checking in on yourself or a loved one, our Reset Companion can help you build the small daily habits — hydration reminders, cool-down routines, medication check-ins — that make hot spells safer.

When is heat a medical emergency?

Heat exhaustion (heavy sweating, clammy pale skin, dizziness, nausea, headache) should improve within 30 minutes with cooling and fluids. If it doesn't — or if the person has hot dry skin, a very high temperature, confusion, seizures, or loses consciousness — that is heatstroke, a medical emergency. Call 999 immediately and cool them while waiting.

The Bottom Line

The 2,700 people who died in England and Wales this May and June didn't, for the most part, die of anything that looked like heat. They died of heart attacks, strokes, kidney failure and respiratory failure — the ordinary causes, arriving early, because heat is a sustained physiological stress test that a compromised body can fail.

That's what makes heat so easy to ignore, and so worth taking seriously. The deaths are real but invisible; the danger peaks not in the dramatic extremes but in the ordinary warm spell nobody worries about; and the people who die are, overwhelmingly, the old, the ill, and the alone.

The corresponding good news is that heat harm is highly preventable, and prevention costs nothing. Keep the home cool. Drink before you're thirsty. Don't exert yourself at midday. Understand your medications. And check on the people who won't ask for help.

The heat is coming back — it always does now, and the science says it's arriving earlier and harder than it used to. The physiology hasn't changed. What can change is what we do about it.

This is general information rather than medical advice. If you have a heart, lung, or kidney condition, or take regular medication, speak to your GP or pharmacist about how hot weather affects you. In a suspected heatstroke, call 999.

Related reading: How to Stay Cool in a Heatwave · Managing High Blood Pressure in the Heat · Do Heatwaves Really Affect Women More Than Men?

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